Proper control of an electrical machine such as an alternator-starter, in particular a variable reluctance alternator-starter, requires that the electrical position of its rotor be known accurately. At present, knowledge of this position is obtained by a position-measuring device using a specific sensor, e.g. with a toothed wheel, that has a number of teeth equal to the number of poles of the rotor of the electrical machine in order to provide the required information over the entire range of speeds of rotation covered by the engine (from stationary (zero speed) to maximum speed) and over all ranges of variation in parameters that have an influence (temperature, voltage of on-board network, . . . ).
Nevertheless, those position-measuring devices which provide good performance and which, in themselves, give full satisfaction, are complex and expensive. In addition, they are mounted in physical association with the engine and transmission unit (in particular at the end of the clutch) together with the existing toothed-wheel sensor for controlling the engine. Although the volume of the specific sensor has been made very small (typically it presents a thickness of about 6 millimeters (mm)), it nevertheless remains that the sensor constitutes an impediment both for mounting the engine and transmission unit in a location where space is short, and for performing any adjustment and/or calibration operation. Furthermore, devices that are mounted in this way are too exposed (heat from the engine, dust, oil, . . . ), which can have an unfavorable influence on their operating conditions and on their lifetime.
There thus exists an urgent need for arrangements for detecting the position of the rotor of an electrical machine that are less expensive than present-day devices and that can be installed away from present mounting locations, in places where more space is available, and above all where they are less exposed to unfavorable conditions.
It should be observed at this point that the toothed-wheel sensor used for engine control and that can deliver an absolute value for the angular position of the crankshaft (a 60-2 toothed sensor in the worldwide standard) has a number of teeth that is not equal to the number of poles in the rotor of the electrical machine.
That sensor can therefore provide a reliable value for the angular position of the crankshaft only above some minimum value for speed of rotation, and in practice a speed of the order of 500 revolutions per minute (rpm).
Such a sensor therefore cannot be used for providing the desired information concerning the angular position of the crankshaft at speeds going from zero speed up to the minimum speed at which the sensor operates properly.
Unfortunately, specifically with an alternator-starter, knowledge of the angular position of the crank-shaft is needed in order to control the alternator-starter when starting the engine, i.e. at speeds that are low and beginning from zero speed.